The present invention relates generally to methods of testing semiconductor devices, and more specifically to a method of determining the carrier concentration depth profile of wide bandgap semiconductors using photoelectrochemical capacitance-voltage measurements.
Testing a wafer sample in order to determine carrier concentration depth profiles using a capacitance-voltage (C-V) technique is well known. This technique includes placing a metal Schottky contact on the sample to be measured and applying a voltage such that a depletion region (region of no carriers) is manifested under the Schottky. Increasing the voltage causes the front of the depletion region to move. At each voltage, a capacitance measurement is made which is used with mathematical manipulation to determine a carrier concentration at an associated depth from the Schottky contact. A shortcoming of this method is that the material being measured can accept only a limited amount of applied voltage before the electric field becomes so high it goes into a breakdown mode. When breakdown occurs the measurement cannot be made. The depth of most materials used for devices is much deeper than can be probed using this technique.
One improved variation on the C-V technique includes etching a sample to a certain depth with a liquid electrolyte that doubles as a Schottky and then making the C-V measurement at each etch point. This method needs to be computer controlled so that the system takes a C-V measurement, then etches to a predetermined depth, then takes another C-V measurement, so that the system cycles down through the material. This allows one to see the carrier depth profile down to an unlimited depth by negating the problem of voltage breakdown.
In order to prevent uncontrolled etching, a photoelectrochemical method was developed so that the Schottky electrolyte would only etch the material when it is exposed to a light source whose energy was larger than the energy bandgap of the material being measured. In the past, this method has been limited to materials with energy bandgaps of about 1.5 eV or less at room temperature.
It is known that wide bandgap semiconductors, by their very nature, are quite promising at high temperatures and recently, much effort has been expended in the development of widebandgap semiconductor materials such as gallium nitride for use in high temperature transistors and blue lasers.
But, a significant problem in the widespread, commercial utilization of wide bandgap materials such as gallium nitride or silicon carbide is the aforesaid 1.5 eV bandgap limitation. More specifically, the 1.5 eV bandgap of the known photoelectrochemical capacitance-voltage method is insufficient to reliably etch wide bandgap materials.
A need exists therefore for an improved photoelectrochemical capacitance-voltage measurement method. Such a method would be useful for reliably determining carrier concentration profiles in wide bandgap semiconductor materials such as gallium nitride and silicon carbide.
It is therefore a primary object of the present invention to provide a method of making photoelectrochemical capacitance-voltage measurements of wide bandgap semiconductors overcoming the limitations and disadvantages of the prior art.
It is another object of the present invention to provide a method of making photoelectrochemical capacitance-voltage measurements providing an accurate carrier concentration depth profile in wide bandgap semiconductor materials.
These and other objects of the invention will become apparent as the description of the representative embodiments proceeds.